1. Field of the Invention
The present invention relates to an amplifier, and more particularly, to an amplifier with increased bandwidth by current injection.
2. Description of the Prior Art
Operational amplifiers are one of the most widely used structures in modern day engineering. They can have many applications ranging from buffers and filters to analog-to-digital converters. Where high performance is required, such as a high speed, high resolution analog-to-digital converter, an operational amplifier having both high gain and high bandwidth is necessary.
Please refer to FIG. 1. FIG. 1 shows a conventional telescopic operational amplifier 10. It comprises a plurality of transistors M1-M9. Matched transistors M1 and M2 serve as an input stage 12 for respectively receiving a non-inverting input Vip and an inverting input Vin, while transistors M3-M8 biased by voltages Vbn, Vbp1, Vbp2 act as a load stage 14 (i.e., an active load). The remaining transistor M9 is biased by voltage Vcmfb, and is the tail transistor acting as a current source for sinking a constant reference current IQ, thereby steering the currents I1 and I2. Since transistors M5 and M6 are a matched pair and transistors M7 and M8 are a matched pair, currents I1 and I2 therefore have the same current value. If both transistors M1 and M2 are turned on, I1 is equal to half IQ, and I1 is equal to I2. As shown in FIG. 1, both the non-inverting output Von and the inverting output Vop are coupled to an output capacitor C0. A telescopic configuration is often a preferred configuration because telescopic operational amplifiers consume less power than other topologies. They also have the added advantage of high speed. The telescopic configuration is well known to those skilled in the art, and therefore is not described in detail here.
The frequency response of an operational amplifier is the response of the circuit in the frequency domain. In other words, if the circuit is given a sinusoidal input the response should be a sinusoidal output of the same frequency, but amplified by the open loop gain, A0. This frequency response can be modeled as a low pass function.
                                          H            ⁢                                                  ⁢                          (              s              )                                =                                    A              0                                      1              +                              s                                  ω                  p                                                                    ⁢                                  ⁢                              where            ⁢                                                  ⁢                          ω              p                        ⁢                                                  ⁢            is            ⁢                                                  ⁢            the            ⁢                                                  ⁢            dominate            ⁢                                                  ⁢            pole            ⁢                                                  ⁢            and            ⁢                                                  ⁢            s                    =                      j            ⁢                                                  ⁢            ω                                              Equation        ⁢                                  ⁢                  (          1          )                    
For frequencies much greater than ωp, i.e. ω>>ωp, we can approximate the gain, A, to be:
                              A          ⁡                      (            jω            )                          =                                            A              0                        ⁢                          ω              p                                jω                                    Equation        ⁢                                  ⁢                  (          2          )                                                                        A            ⁡                          (              jω              )                                                =                                            A              0                        ⁢                          ω              p                                ω                                    Equation        ⁢                                  ⁢                  (          3          )                    Therefore, putting this result back into equation (1), we can obtain the unity-gain bandwidth ωu:ωu=A0ωp  Equation (4)
The unity gain bandwidth can also be written in terms of device transconductance, gm, and output resistance, r0. Equation (4) then becomes:
                              ω          u                =                                            gm              ×                              r                0                                                                    r                0                            ⁢                              C                0                                              =                      gm                                          C                op                            +                              C                load                                                                        Equation        ⁢                                  ⁢                  (          5          )                    
From equation (5) it can be seen that the unity gain bandwidth ωu is a function of device transconductance gm, and total output capacitance C0. The total output capacitance includes parasitic junction capacitance Cop, and capacitive load Cload driven by the operational amplifier 10. Therefore, the way to increase bandwidth is by increasing gm or by decreasing the output capacitance C0. Cload cannot be decreased because it is determined by circuit specification and hence cannot be decreased for a specific application. Furthermore, the parasitic capacitance dominates the capacitive load, so decreasing Cload would only have a minimal effect. To increase gm would require a larger device, which then leads to more parasitic capacitance, as the device size cannot be increased without limits. This gain-bandwidth relation therefore creates an upper level bandwidth limit.